1. Field of the Invention
The present invention relates to a semiconductor junction capacitance element such as variable capacitance diode or the like which is resistant to electrostatic breakdown.
2. Description of the Prior Art
Generally, among methods of preventing electrostatic breakdown of a semiconductor device are a method in which waveform of surge voltage resulting from static electricity is smoothed out by means of a time constant defined by a capacitor and resistor, thereby preventing an abnormally high potential from being applied to the internal circuit of the semiconductor device, a method in which a protective diode is employed, and a method in which a discharge tube is utilized. Commonly, a diode can readily be mounted onto a printed circuit board or incorporated in the semiconductor device, and thus has widely been employed for the purpose of preventing electrostatic breakdown of a semiconductor device.
Referring to FIG. 1 of the accompanying drawings, there is shown, in cross-section, an example of the prior-art semiconductor device incorporating diodes for preventing electrostatic breakdown. In FIG. 1, a semiconductor substrate 1 comprises a semiconductor substrate 2 of the N.sup.++ conductivity type (lower resistivity) and an epitaxial layer 3 of the N.sup.- conductivity type (higher resistivity). In the epitaxial layer 3, there are provided diffusion layers 5 and 7 of the N.sup.+ conductivity type (low resistivity) and P conductivity type diffusion layers 6 and 8 which overlie the diffusion layers 5 and 7 respectively in such a manner as to define PN junctions J.sub.1 and J.sub.2 which constitute a variable capacitance diode and a diode for preventing electrostatic breakdown respectively. A conductive film 9 is provided on the main surface of the diffusion layers 6 and 8. Indicated at 4 is a silicon dioxide film. The PN junction J.sub.2 defined by the N.sup.+ conductivity type diffusion layer 7 and the P conductivity type diffusion layer 8 is so established as to have a lower breakdown voltage than the PN junction J.sub.1. If a surge voltage or the like is applied to the conductive film 9, then discharge will be caused to occur through the protection diode J.sub.2 for preventing electrostatic breakdown so that the PN junction J.sub.1 will be prevented from electrostatic breakdown.
However, the conventional semiconductor junction capacitance element is disadvantageous in that in the case where a protection diode is connected in parallel therewith to prevent electrostatic breakdown, the junction capacitance of the protection diode is added to the capacitance of the semiconductor junction capacitance element which is dominant, so that the dominant capacitance is changed, and thus such a diode cannot be used when it is attempted to achieve precise junction capacitance.
A variable capacitance diode for UHF, for example, has capacitance of several pF, so that when a protection diode is connected in parallel therewith, the junction capacitance of the protection diode turns out not to be negligible and thus no diode can be employed for the purpose of preventing electrostatic breakdown.
Another method of preventing electrostatic breakdown for such an application is to mount a discharge tube onto a printed circuit board to thereby protect the internal circuit from surge voltage. Disadvantageously, however, the discharge tube is expensive and bulky. Another disadvantage is that from the standpoint of its life span, the discharge tube is unsuitable for portable communication devices.